The present invention relates to a system for detecting vibrations from a remote surface and to a method of using same. Embodiments of the present invention relate to a system which utilizes focused speckle imaging to detect vibrations from a remote object or subject.
Devices for measuring surface vibrations of a remote object are known in the art. For example, Laser Doppler Vibrometers (LDV) measure the Doppler shift of a laser beam reflected from a surface to extract the surface velocity and determine surface vibrations.
Devices for measuring speckle patterns generated on a surface are also utilized for identifying surface vibrations of remote objects. Since speckles are characterized by an intensity pattern produced by mutual interference of a set of wave fronts, typical speckles analysis utilizes out-of-focus speckle images in order to maximize the amount of information that can be obtained from this pattern over time. Defocused speckles imaging provides detailed images of the speckles that allows tracking their variation over time to extract surface motion such as rotation, translation and deformation.
Both LDV and speckle analysis approaches suffer from inherent limitations. With LDVs, rough surfaces generate speckles in the reflected light field that generate random noise in the measurement. As a result, surfaces measured by LDVs are usually treated to provide specular reflectance when possible. In addition, LDVs are complicated devices that require an expert for proper operation and utilize laser power which exceeds eye-safety limitations.
With speckle analysis, the camera frame rate limits the frequency band of the extracted information. Although speckle analysis approaches can utilize a high-speed camera (tens of KHz), a large number of frames must be captured, saved in memory and analyzed limiting real-time performance and the size of the measurement time window. In addition, in order to capture meaningful information defocused speckle imaging must cover a relatively large number of pixels. While strong defocusing spreads the laser spot image over multiple pixels, it results in a substantial drop in light intensity which is compensated for by increasing the power of the laser source oftentimes beyond eye safety range.
Although the above solutions can provide quantitative information with respect to a surface, some applications do not require such quantitative information. For example, various applications in modern life require automatic detection of the presence of subjects within an area of interest. In such applications, the main requirements are accurate identification of a subject or subjects without utilizing potentially unsafe radiation and not quantitative information regarding a surface.
Thus, it would be highly advantageous to have, a system which can be used to accurately detect a presence and number of subjects in an environment without subjecting the subjects to potentially harmful radiation.